feat: 添加管理员权限管理功能，更新 AdminUser 模型以支持权限加载，扩展 API 路由以管理后台用户及其权限

database/migrations/2026_05_11_173000_create_admin_user_permissions_table.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('admin_user_permissions', function (Blueprint $table) {
+            $table->foreignId('admin_user_id')->constrained('admin_users')->cascadeOnDelete();
+            $table->foreignId('permission_id')->constrained('admin_permissions')->cascadeOnDelete();
+            $table->primary(['admin_user_id', 'permission_id']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('admin_user_permissions');
+    }
+};